The present invention relates to an electrostatographic apparatus such as an electrostatic copying machine. It is desirable for an electrostatic copying machine to produce good copies of all types of original documents including printed pages and photographs. However, such copying machines known heretofore have generally been able to produce good copies only of printed pages. When these copying machines produce copies of photographs having large dark areas, the dark areas tend to appear washed out and the copy contrast is very low. Such copies are completely unsatisfactory.
It is known in the art to control a bias voltage applied during development in accordance with the sensed potential of an electrostatic image of an original document formed on an electrostatic drum or belt. The bias voltage is adjusted to be equal to or slightly above the potential of the background areas of the electrostatic image. This prevents toner transfer to the background image areas during development and ensures clean white background areas in the copy.
The background areas of the electrostatic image will not have zero potential even if the background areas of the original document are pure white. The background areas of the electrostatic image will have a residual potential which is a function of fatigue of the photoconductive coating on the drum or belt, deterioration of an imaging light source, contamination of mirrors and lenses in an exposure optical system and the like. In addition, due to the spectral sensitivity of the drum the background electrostatic potential for electrostatic images of documents having different colored backgrounds will differ as a function of the color.
The known system is generally operative to adjust the developing bias voltage to an optimum value regardless of all variations in the above mentioned parameters where the document is a printed page. However, photographic documents or documents having white printing on dark backgrounds are not reproduced satisfactorily with such a system. The bias voltage will be set to an excessive value causing the dark areas to be washed out and the contrast to be extremely low.
A proposed expedient to allow copying of photographic documents is disclosed in Japanese patent application no. 49-81884 and patent application publication no. 51-950. This expedient is to limit the maximum bias voltage to a predetermined value. Although it improves the copy quality and reduces the washing out of dark image areas in copies of photographic documents, the copies produced by such a system are still not faithful reproductions of the original documents.